gcc: disable '-Warray-bounds' for gcc-9

'-Warray-bounds' is already disabled for gcc-10+.  Now that we've merged
bitmap_{read,write), I see the following error when building the kernel
with gcc-9.4 (Ubuntu 20.04.4 LTS) for x86_64 allmodconfig:

drivers/pinctrl/pinctrl-cy8c95x0.c: In function `cy8c95x0_read_regs_mask.isra.0':
include/linux/bitmap.h:756:18: error: array subscript [1, 288230376151711744] is outside array bounds of `long unsigned int[1]' [-Werror=array-bounds]
  756 |  value_high = map[index + 1] & BITMAP_LAST_WORD_MASK(start + nbits);
      |               ~~~^~~~~~~~~~~

The immediate reason is that the commit b4475970 ("bitmap: make
bitmap_{get,set}_value8() use bitmap_{read,write}()") switched the
bitmap_get_value8() to an alias of bitmap_read(); the same for 'set'.

Now; the code that triggers Warray-bounds, calls the function like this:

  #define MAX_BANK 8
  #define BANK_SZ 8
  #define MAX_LINE        (MAX_BANK * BANK_SZ)
  DECLARE_BITMAP(tval, MAX_LINE); // 64-bit map: unsigned l...